The development of integrated circuits has made it possible to place many circuit elements in a single semiconductor chip. Where part or all of the circuit is an analog circuit, such as a radio frequency transmitter or receiver, audio amplifier, or other such circuit, circuit design requires lumped elements that cannot be readily realized in monolithic integrated circuits. Capacitors in particular are frequently created as separate elements from the integrated circuit. The electronic device thus typically includes monolithic integrated circuits combined with external capacitors.
For such applications, monolithic ceramic capacitors have been used. For example, single capacitors made of ceramic materials, are known in the art. These are relatively small in size and can be surface mounted to a surface mount circuit board, or glued and wire bonded to a substrate in a hybrid circuit layout.
In an ideal model of a lumped element capacitor, the capacitor provides an ideal voltage/current relationship:
  i  =      C    ⁢                  ⅆ        v                    ⅆ        t            Unfortunately, particularly at high frequencies, capacitors used in electronic circuits deviate substantially from this ideal relationship. These deviations are generally modeled as an equivalent series resistance and equivalent series inductance, along with a capacitance that varies over frequency. In accordance with this model, a capacitor behaves as a series L-R-C circuit. At lower frequencies, the dominant impedance is the capacitive element C. However, at increasing frequencies, the impedance of the capacitive element C decreases; and the impedance of the inductive element L increases. Then, at the resonant angular frequency (LC)−0.5, the inductive element becomes predominant; and the element ceases performing as a capacitor. Simultaneously, the capacitor dissipates some stored energy (typically through heating of conducting plates and traces), as represented by the series resistance R.
Capacitor design typically must compromise between capacitance value and equivalent series resistance and inductance; greater capacitance typically can be created only at the cost of increased series resistance and inductance. Accordingly, equivalent series resistance and inductance are not avoidable, and electronic design must take them into account, particularly in high frequency products such as broadband receiver/transmitters, short wave devices, and the like.
Various monolithic ceramic structures have been developed to provide relatively small capacitors for highly integrated applications. A first such structure is known as a “multilayer ceramic capacitor”. This structure is formed by stacking sheets of green tape or greenware, i.e., thin layers of a powdered ceramic dielectric material held together by a binder that is typically organic. Such sheets, typically, although not necessarily, are of the order of five inches by five inches, can be stacked with additional layers, thirty to one hundred or so layers thick. After each layer is stacked, conductive structures are printed on top of the layer, to form internal plates that form the desired capacitance. When all layers are stacked, they are compressed and diced into capacitors. Then, the compressed individual devices are heated in a kiln according to a desired time-temperature profile, driving off the organic binder and sintering or fusing the powdered ceramic material into a monolithic structure. The device is then dipped in conductive material to form end terminations for the internal conductive structures, suitable for soldering to a surface mount circuit board or gluing and wire bonding to a hybrid circuit.
The design of known broadband capacitors involves a tradeoff between capacitance value and broadband performance. One known approach to managing series resistance and series inductance, is to parallel connect a multilayer capacitor with a single-layer capacitor. The larger value capacitor is chosen for its large capacitance value and is parallel connected to the smaller value capacitor that is chosen for its small equivalent series resistance. As will be appreciated, such a circuit exhibits multiple resonant frequencies, a first at the frequency (L1C1)−0.5, and a second at the frequency (L2C2)−0.5. Typically the larger valued capacitor has the larger series resistance and inductance value and thus, the lower resonant frequency. The smaller valued capacitor is chosen for high frequency performance resulting from low series resistance and series inductance values. At lower frequencies, the larger capacitor will produce acceptable performance; however, at higher frequencies, where the larger capacitor behaves increasingly less like a capacitor and more like an inductance, the smaller capacitor will be below its resonant frequency and perform well as a capacitor throughout the frequency of interest.
The parallel capacitor approach has been utilized in conjunction with ceramic chip capacitors to improve the high frequency performance of those capacitors. Multilayer and single-layer capacitor combinations are often designed to utilize surface mount technologies; and therefore, the capacitor terminal plates or contacts are on opposed upper and lower sides of the capacitor. In applications where it is desirable to use wirebonding connections, it is necessary to provide electrical connections with a wire bonded to an upper contact. As shown in FIG. 4, an integrated broadband capacitor 18 has a multilayer capacitor 20 with sets of opposed and parallel plates 21, 23 disposed in a ceramic dielectric body 25. Each set of plates 21, 23 is electrically connected to a different one of the conductive contacts 22, 24 on opposite sides of a ceramic dielectric body 26 in a known manner. A higher frequency, single-layer capacitor 28 is formed from opposed plates 30, 32 that also serve as end contacts, with the contact 32 being electrically connected to a conductor 34. The multilayer capacitor 20 is connected in parallel with a single-layer capacitor 28 to provide an equivalent circuit shown in FIG. 4A. A connecting wire 36 connects an integrated circuit (“IC”) 38 with the contact 30 of the single-layer capacitor 28. The connecting wire 36 is relatively long; and the inductance of the wire 36 increases loss in the system, thereby adversely affecting its performance.
Referring to FIG. 5, another known integrated broadband capacitor 40 made for wirebonding also has an equivalent circuit of parallel connected capacitors as shown in FIG. 4A. The integrated capacitor 40 also has the multilayer capacitor 20 identical to the multilayer capacitor 20 of FIG. 4. The multilayer capacitor 20 is connected to a double single-layer capacitor 42. The double single-layer capacitor is a substrate-like piece containing two single-layer capacitors. A first capacitor 44 is formed between plate contacts 46, 48. Plate contact 46 is electrically connected by the wire 36 to IC 38, and plate contact 48 is electrically connected to conductor 34. Contact plates 50, 52, which would normally form a capacitor therebetween, are shorted with a silver paste 54 that is fired at about 800 degrees C. A disadvantage of using such a capacitor 42 is that it is difficult to handle a 5 mil thick ceramic device in order to dip the plates 50, 52 in the silver paste; and often, the device is broken in the dipping process.
Thus, there is a need for a multilayer and single-layer broadband capacitor suitable for use with wirebonding that can be produced using existing automated production equipment and processes and does not require special handling and operations.